1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device with a tablet for entering information by touching the finger for example on the surface of the LCD device, namely by detecting a coordinate position, the LCD device being for displaying images, characters, symbols, numbers and other information.
2. Description of the Related Art
Conventionally, the liquid crystal display (LCD) device attached with an input mechanism called a tablet on the front of the display screen is in a wide use on banking online systems and dedicated Japanese wordprocessors for example. This input mechanism has been implemented in several manners including the resistance type in which a resistance change caused by touching of the finger for example on the tablet is sensed as a switching operation, the electrostatic capacity type in which change of the electrostatic capacity between electrodes by the finger touching is sensed as a switching operation, the optical type in which light is blocked by the finger for example to cause a switching operation, and the magnetostriction type in which a magnetic change caused by the finger for example is sensed as a switching operation.
The tablet of the electrostatic capacity type is constituted by a pair of transparent base plates formed with electrodes for electrostatic capacity detection, the pair of transparent base plates being integrally laminated with a transparent insulation film in between. The tablet thus constituted is adherently attached on the front surface of the LCD. However, such a setup reduces the transmittance of liquid crystal display due to the tablet insulation film and the ITO electrodes providing detection electrodes. Besides, because the tablet is adherently attached on the display screen, an offset may occur between them. Consequently, a position on the tablet which the finger touches and a corresponding position on the display screen are shifted, reducing the visibility of the display on the LCD.
To overcome those problems, a technique was disclosed in Japanese Patent Laid-Open No. Hei 5-19233 (1993), in which the upper glass plate on the LCD also serves as the base plate of the tablet, reducing the number of components necessary for constituting the tablet and, at the same time, reducing the thickness of the tablet.
In the above-mentioned disclosed LCD device with a tablet, the device may be made thinner to some extent; however, because the tablet and the LCD device are electrically independent of each other, a driver circuit dedicated to the tablet is required to increase the fabrication cost.